docs: improve documentation of OptimizeConn (#3910)

sys_conn.go

@@ -25,12 +25,16 @@ var _ OOBCapablePacketConn = &net.UDPConn{}
 
 // OptimizeConn takes a net.PacketConn and attempts to enable various optimizations that will improve QUIC performance:
 //  1. It enables the Don't Fragment (DF) bit on the IP header.
-//     This allows us to do DPLPMTUD (Path MTU Discovery).
+//     This is required to run DPLPMTUD (Path MTU Discovery, RFC 8899).
 //  2. It enables reading of the ECN bits from the IP header.
 //     This allows the remote node to speed up its loss detection and recovery.
 //  3. It uses batched syscalls (recvmmsg) to more efficiently receive packets from the socket.
+//  4. It uses Generic Segmentation Offload (GSO) to efficiently send batches of packets (on Linux).
 //
-// For this to work, the connection needs to implement the OOBCapablePacketConn interface (as a *net.UDPConn does).
+// In order for this to work, the connection needs to implement the OOBCapablePacketConn interface (as a *net.UDPConn does).
+//
+// It's only necessary to call this function explicitly if the application calls WriteTo
+// after passing the connection to the Transport.
 func OptimizeConn(c net.PacketConn) (net.PacketConn, error) {
 	return wrapConn(c)
 }

transport.go

@@ -31,7 +31,8 @@ type Transport struct {
 	// Bad things will happen if passed to multiple Transports.
 	//
 	// If not done by the user, the connection is passed through OptimizeConn to enable a number of optimizations.
-	// After passing the connection to the Transport, its invalid to call ReadFrom and WriteTo.
+	// After passing the connection to the Transport, it's invalid to call ReadFrom on the connection.
+	// Calling WriteTo is only valid on the connection returned by OptimizeConn.
 	Conn net.PacketConn
 
 	// The length of the connection ID in bytes.